1. Field of the Invention
The present invention relates to adhesive sheets suitable for use in the construction of circuit boards and other electrical devices.
2. Description of Related Art
Multi-layer Printed Circuit boards are presently made out of two basic dielectric components. The circuitry is etched onto copper clad dielectric substrates that make up one of those components. The other component, referred to as a prepreg, bond ply, or bonding film, is an adhesive layer that is used to attach the etched substrates together to form the multi-layer structure.
Traditionally, a woven glass reinforced epoxy has been the material of choice for multi-layer printed circuits. The material is easy to work with, inexpensive, and has fairly good electrical properties. In areas that require higher performance, like high speed digital or microwave applications, the electrical properties of the epoxy are not sufficient and an alternative must be used.
The highest performance of the low dielectric substrates currently available are made from a composite of polytetrafluoroethylene (PTFE) and glass or ceramic. Though this material is difficult to process, its excellent electrical properties make it ideal for many of the higher frequency circuits (above 400 MHz). Due to required high pressures and temperatures for processing into a composite, this material has gained wide acceptance as a copper clad substrate, but only limited acceptance as a bond ply.
The second component of these high performance, multi-layer circuits, the bond ply, has traditionally been a thermoplastic film that is much easier to process and requires much lower pressures and temperatures than bonding with the PTFE. The high temperatures and pressures associated with PTFE bonding or "sintering" can be very inconvenient, and often impossible to employ. Additionally, such extreme processing introduces large amounts of thermal and mechanical stress into the circuit board. In addition, filled PTFE materials have poor flow characteristics which limit the ability of the bonding film to conform around the etched circuitry on the substrates being bonded together. Though thermoplastic bond plies have advantages over PTFE bonding by reducing processing temperatures and pressures while increased flow characteristics, they have several inherent limitations.
One limitation of thermoplastic bonding films is that they do not match the dielectric constant of the substrates that they are bonding together. This difference in properties causes problems for very high frequency and high performance circuits. The propagating electromagnetic signal is not only affected by the overall dielectric constant (Dk) of the PWB, but is also strongly affected by local changes of the dielectric constant. An improperly Dk-matched system leads to unreliable signal propagation and unreliable signal integrity.
Another limitation is that the nature of thermoplastics is such that they will always flow at a given temperature. Multi-layer circuits that require a sequential lamination operation (i.e., more than one bonding operation) require the use of different thermoplastic bonding films. This is needed because if the temperature of the previous bond film is reached, then that previous joint will reflow, causing mis-registration of the inner layer substrates.
This may lead to another limitation. There are only a few thermoplastic bonding films on the market that operate at different temperatures and meet the electrical requirements of these high performance circuits. This limits the number of sequential laminations that can be done for a given circuit. The lowest temperature of these films bonds at 250.degree. F., but loses about 85% of its strength at 180.degree. F. That means that if the finished circuit is going to need to go through high temperature operations like soldering, then there are only a couple of bonding film options. It would be desirable, therefore, to have a system that would allow for sequential laminations and not have the same temperature limitations of the finished circuit.
Another common problem with these materials is that the thermoplastics have coefficients of thermal expansion (CTE) that are three to four times as high as the CTE of the substrate. Failure to match these characteristics properly often leads to excessive stress within the board, premature separation of the adhesive layer, inconsistent electrical performance, and unreliable plated-through-holes.
Despite these limitations, thermoplastic bond plies continue to be used because no acceptable substitute has been found. The function of this component is to hold the substrates of a multi-layer circuit together while providing the necessary electrical and physical properties to produce an optimized circuit. While thermoplastic systems have problems with unmatched Dk and poor processing, thermoset systems have had problems with unmatched Dk, high dielectric loss, and poor adhesion to the PTFE substrates.
U.S. Pat. No. 4,518,737 to Traut describes an isotropic composite of a fluoropolymer binder and ceramic filler, which contains both microfiber and particulate, especially in the range of 10 and 75 weight percentage. U.S. Pat. No. 4,849,284 to Arthur describes electrical substrates comprising fluoropolymer and a ceramic filler where the filler is at least 55% by weight. Although these composites may display good electrical and dimensional stability, making them good copper clad substrate materials, they are difficult to process and display markedly difficult rheology for flow and fill into the inter spacings of a circuit board. Though a bond ply made of the same material would have a good Dk match, it would have poor flow characteristics and undesirable processing conditions. They are poor bond films.
A number of further refinements to this basic technology have been proposed in subsequent patents, but none is believed fully satisfactory. For example: zirconate surface treatment is taught in U.S. Pat. No. 5,024,871 to Arthur et al.; silane surface treatment on a ceramic filler is taught in U.S. Pat. No. 5,149,590 to Arthur et al.; fluoropolymer and ceramic composites with a silane coating in the range of 45-50% by volume percent is taught in U.S. Pat. No. 5,194,326 to Arthur et al.; a titanate or zirconate filler is taught in U.S. Pat. No. 5,198,295 to Arthur et al.; and a bond-ply of ceramic-filled fluoropolymer with a ceramic coated with a coupling agent is taught in U.S. Pat. No. 5,281,466 to Arthur et al. Among the deficiencies with each of the above products are believed to be only slightly improved rheological flow to fill gaps and traces within inner layers of the circuit boards (i.e., they are still poor bond films). In the case of U.S. Pat. No. 5,281,466, lower filler levels are used to increase the flow characteristics, but at the expense of the Dk and CTE match.
Moreover, in each of the prior examples, the method for making a useful article is by bonding the PTFE in said composites at so-called "sintering" temperatures (i.e., very high temperatures and pressures on the order of above 662.degree. F. (350.degree. C.) and 1000 lb/in.sup.2 (678 N/cm.sup.2). As previously discussed, this difficulty in processing has prevented these composites from gaining widespread use as a bond ply.
In U.S. Pat. No. 4,985,296 to Mortimer, it is taught that an expanded PTFE film 0.1 to 5.0 mil thick can be filled with an inorganic filler while remaining substantially pin hole free. U.S. Pat. No. 4,996,097 to Fischer teaches similar technology useful for a thin capacitive layer in a printed circuit board. While these products have better flow characteristics, they still maintain the high pressures and temperatures that make them undesirable as bond plies.
All of the above examples teach methods of producing a material with good electrical and physical properties, but they all maintain the high temperatures and pressures that preclude them from having the desirable processing for a bonding film.
Thermosetting resins have traditionally provided ease of processing in making multi-layer circuits. An approach to making a bonding film involves using various fillers that have been added to thermosetting adhesives to improve performance characteristics, particularly with regard to matching coefficients of thermal expansion of component parts. For example, silicon dioxide (SiO.sub.2) particles are commonly employed to provide a variety of improved performance characteristics, such as lower coefficient of thermal expansion (CTE) and more desirable shrinkage performance.
Unfortunately, even the filled adhesive approach to bonding films has a number of performance and handling deficiencies. The resins used in combination with the fillers provide good adhesion, but have very poor electrical performance. Loading the adhesive with filler makes the adhesive film brittle and difficult to handle. Therefore, to date there has been no practical way to deliver the desired properties with a filled thermosetting resin.
To aid in the application of adhesives, a number of adhesive materials have been incorporated into a carrier sheet to form an adhesive "bond film." For instance, W. L. Gore & Associates, Inc., sells a bond film material under the trademark SPEEDBOARD that comprises an expanded polytetrafluoroethylene (PTFE) matrix (such as that made in accordance with U.S. Pat. No. 3,953,566 to Gore) filled with an adhesive resin material. This product forms a good bond for most applications and is easy to handle and apply. Nevertheless, further performance improvements remain of interest, particularly with regard to better matches in dielectric constant at microwave frequencies and the coefficients of thermal expansion.
In UK Patent 2,195,269 to Hatakayama et al. a composite of porous PTFE substrate with resin imbibed within its node-and-fibril structure is taught. While this patent also mentions that an inorganic filler may be incorporated in the expanded, porous PTFE, the ranges of fill taught by the patent (i.e., about 25-45% volume ceramic and about 25-40% volume adhesive) have proven ineffective in addressing the Dk match, flow characteristics and adhesion necessary to make an acceptable bonding film for PTFE substrates.
The Fisher patent also teaches that resin can be imbibed into a ceramic filled PTFE structure, but again, the product in that form would still not have the Dk match, flow characteristics, and adhesion necessary to make an acceptable bonding film for PTFE substrates.
Despite previous attempts to produce suitable material, no prior composite has provided all of the desired properties and processability for any given combination of ceramic (or other fillers), PTFE and resin.
Accordingly, it is a primary purpose of the present invention to provide an adhesive material that is dimensionally stable and has a relatively matched dielectric constant with its substrate (or "core") material.
It is a further purpose of the present invention to provide an adhesive bond film material that can be processed at relatively low temperatures and pressures so as not to introduce additional stress into a circuit board or other electrical structure.
It is still further the purpose of the present invention to provide an adhesive bond film material that allows for a successive fabrication of multi-layer PWB for microwave applications.
These and other purposes of the present invention will become evident from review of the following specification.